mineral

Chalcedony (Agate/Chert)

Silicon Dioxide (SiO₂)

Hardness: 6.5–7 on the Mohs scale; Color: waxy tan, honey-yellow, to milky white; Luster: waxy to vitreous; Crystal structure: Trigonal/Monoclinic (cryptocrystalline); Cleavage: none; Fracture: conchoidal; Specific gravity: 2.6–2.64.

Hardness: 6
Color: waxy tan, honey-yellow, to milky white
Luster: waxy to vitreous

Identified April 8, 2026More mineral →

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Physical properties

Formation & geological history

Formed from the deposition of silica from groundwater in cavities or veins of igneous rocks or as nodules in sedimentary rocks. Often found in volcanic environments or limestone beds; age can range from relatively recent to hundreds of millions of years.

Uses & applications

Used primarily in lapidary work for jewelry, ornaments, and carvings. Historically used for making tools and arrowheads due to its conchoidal fracturing. Also used in industrial applications for precision bearings.

Geological facts

Chalcedony is a cryptocrystalline form of silica, meaning its crystals are too small to be seen even under a microscope. It has been used for seals and cylinder seals since ancient Mesopotamian times (c. 4000-3000 BC).

Field identification & locations

Identify in the field by its waxy luster, smooth/glassy conchoidal fracture surfaces, and extreme hardness (it will scratch glass). Often found in desert 'pavement' or dry riverbeds in the American Southwest.